In cellular systems, each geographical cell is served by a separate base station that provides wireless telecommunications services to station sets located within the cell. It is frequently desirable or necessary that the wireless, over-the-air, transmissions and receptions by the plurality of base stations be synchronized with each other. For example, in broadcast systems, synchronization of base stations' transmissions is necessary to prevent the transmissions from adjacent cells being garbled along cell boundaries. In frequency-hopping mobile systems, synchronization of the frequency-hopping by the base stations is necessary so that a station set crossing from one cell to another can be "handed over" by the base station of the exited cell to the base station of the entered cell and continue operating during the hand-over without interruption of communications. And in cellular systems, such as Code-Division Multiple-Access (CDMA) systems, that employ the "soft handoff" technique wherein a station set moving from one cell to another is, for a time, in communication with both cells' base stations, synchronization of the base stations is necessary to permit the simultaneous communication by the station set with a plurality of base stations.
The standard technique for synchronizing the operations of a plurality of base stations and keeping them synchronized is to supply the base stations with a synchronization signal. A number of techniques exist for supplying a synchronization signal to base stations, but all have attendant disadvantages. One technique uses an Earth-orbiting satellite to broadcast a synchronization signal to the base stations. However, this technique is rather complex, in that it requires both the presence of a satellite, and of a satellite signal receiver in each base station. Another technique uses wire or optical fiber links separate from the normal, customer traffic, communications links to connect the base stations to a central controller, over which links the central controller transmits the synchronization signal to the base stations. However, this technique requires the expense of a dedicated synchronization link leading to each base station. In yet another technique, a central controller periodically stops the normal communications on the normal communications links leading between the base stations and the central controller, then broadcasts a synchronization signal on the links, and then resumes the normal communications. While avoiding the expense of separate dedicated synchronization links, this technique causes periodic interruptions in normal communications.
What the art lacks is a simple and inexpensive technique for supplying base-station synchronization signals to a plurality of base stations.